1. Field of the Invention
The present invention relates generally to power switching circuits and, more particularly, to a new improved reverse phase control power switching circuit.
2. Description of the Related Art
It is known to control the phase of the voltage and/or current applied to a load from an Alternating Current source by means of an electronic switch that is switched to its conductive state at a time subsequent the zero crossing of the AC periodic waveform, and to then assume an off or blocking state at the next occurring zero crossing of the waveform when the current through the load decays to zero. A disadvantage with this type of phase control circuit is that the load is subjected to a sudden and abrupt increase in voltage/current when the switching device is turned on. This sudden and abrupt increase in voltage/current to the load can be objectionable. For example, where the load is an incandescent lamp, this surge in current through the lamp filament can create a large magnetic field which can cause the filament and its support to change their lengths to cause what is known as incandescent hum. Clearly, when the incandescent lamp is located in a very low ambient sound environment, the audible sound that is produced can be objectionable.
As noted in the prior art, circuits have been developed which use a current choke such as a large inductor to limit the current rise to an acceptable level to minimize the effects of the abrupt increase in current and voltage over a short period of time. The inductor is normally placed in series with the load to limit the rate of change of current passing through the load to reduce the lamp hum. A disadvantage with this method is that the inductor carries the full load current which causes a reduction in power applied to the load. The voltage drop across the inductor can be as large as several volts. In addition, the inductors can be relatively expensive, undesirably large and may produce objectionable audible hum from their cores.
Power control systems are subject to a variety of conditions since a variety of power sources and loads can be coupled through the switches. Power control switching circuits are subject to damage or performance impairment by a variety of device threatening conditions. These include excessive currents, voltages and temperatures. Accordingly, power control systems using voltage controlled switches include protective circuits that handle these conditions. In some cases these circuits shut down operation until the condition is removed. An example of a device threatening condition is an excessive current, an over current that is caused, for example, by a short circuit or a large load. Such an excessive current can destroy the power switching circuit. Therefore, some over current protection, generally a current limiter, is provided in a power control system. Various circuits and method directed toward addressing these problems have been disclosed in U.S. Pat. Nos. 4,528,494 ('494); 5,004,969 ('969); and, 5,239,255 ('255). U.S. Pat. Nos. '494 and '255 are directed toward protecting reverse phase control power switching circuits from an over current which occurs when an incandescent lamp goes from a non-incandescent state to an incandescent state. Typically, surge currents through a cold filament are over 10 times the steady state requirements when a non-incandescing incandescent lamp is energized. In '494 and '255, the duration of current flow through the load, and the amount of power dissipated therein is gradually increased an the resistance of the cold tungsten filament increases. U.S. Pat. No. '969 discloses a reverse phase control system that can pass current through a load from an AC source without the need for zero-crossing detection of the voltage waveform.
Thus the prior art provides a variety of schemes for addressing device and performance threatening conditions. However, these schemes generally involve undesirable tradeoffs. What is needed is a power control system that provides the necessary protective measures without minimizing performance.